PB1973: INHIBITION OF LACTATE EXPORTERS INDUCES DIRECT ANTI-TUMOR EFFECTS AND COUNTERS IMMUNE SUPPRESSION IN MULTIPLE MYELOMA
Authors: A. Van der Vreken, I. Oudaert, A. Maes, E. De Bruyne, K. De Veirman, K. Vanderkerken, E. Menu
Journal: HemaSphere (2022)

Background: Multiple myeloma (MM) is a cancer characterized by the accumulation of malignant plasma cells in the bone marrow (BM). The BM is an immune suppressive environment, which hampers the efficacy of modern immune therapeutic strategies. Immune suppression is partially orchestrated by the hypoxic nature of the BM. Due to this hypoxia, cancer cells are pushed towards aerobic glycolysis, during which pyruvate is converted to lactate. Lactate can be imported and exported by monocarboxylate transporters (MCTs). Lactate in the BM milieu contributes to an immune suppressive environment by several mechanisms including driving differentiation and attraction of myeloid derived suppressor cell (MDSC). Aims: In this study, we aimed to evaluate the effects of MCT inhibition on MM cell viability and the immune suppressive character of the BM. Methods: Correlation of MCT-expression with survival was investigated using the Mulligan cohort. In vitro, several MM cell lines (LP-1, RPMI-8226, 5TGM1) were used. Syrosingopine was used as dual inhibitor of MCT1 and MCT4, AZD0095 as MCT4 inhibitor and AZD3965 as MCT1 inhibitor. The effect on apoptosis was quantified by AnnexinV/7-AAD staining. For MDSC differentiation, BM was isolated from naïve mice. CD11b+ cells were selected by MACS and cultured with conditioned medium (CM). CM was prepared from 5TGM1 cells treated with DMSO or syrosingopine, for 48 hours in hypoxia (1% O2). The effect on MDSC differentiation was evaluated by western blotting. Results: High expression of MCT1/MCT4 genes correlated with a significant lower overall survival in MM patients. Increased expression of MCT1 and MCT4 was observed in MM cell lines after 48 hours of hypoxia. Syrosingopine treatment resulted in intracellular lactate accumulation but did not induce apoptosis. Neither did single inhibition of MCT1 or MCT4. MM cells could however be sensitized to syrosingopine by treatment with metformin, a complex I inhibitor, resulting in apoptosis. AZD0095 and AZD3965 were less potent to induce cell death in combination with metformin, compared to dual MCT inhibition. The effects of the combination treatment on metabolism and cellular pathways are currently investigated. Extrinsic effects of export inhibition were evaluated by using CM of MM cells treated with syrosingopine on MDSCs. Both lactate and MM-CM promoted MDSC differentiation in vitro, while CM deprived of lactate resulted in a clear decrease of differentiation markers such as p-stat3 and arginase. This indicates a less suppressive status of the MDSC’s. Summary/Conclusion: Dual inhibition of MCT1 and MCT4 is not sufficient to induce apoptosis in MM cells; however, they can be sensitized to syrosingopine with metformin. Blocked lactate export from MM cells resulted in impaired MDSC differentiation. Targeting lactate metabolism could be a new approach to hamper MM survival and create a less suppressive immune environment. Arne Van der Vreken and Inge Oudaert contributed equally to this work.